Typical of the processes and apparatuses described in the aforementioned copending applications is the fact that a flue gas, especially a flue gas from an electricity-generating power plant, can be scrubbed in a scrubbing column or tower with a scrubbing liquid containing a calcium compound or calcium ions so that sulfur oxides are absorbed from the flue gas in a desulfurization operation.
In general, the scrubbing liquid is collected in a sump of the column and, with the calcium compounds and/or calcium ions, is recirculated to the scrubbing zone above the sump which can be traversed by the gas in a counterflow to the scrubbing liquid.
An oxidation zone can be provided in the sump to convert calcium sulfite and calcium bisulfite, formed by the absorption of sulfur dioxide, into calcium sulfate so that the solids which tend to come out of solution in the sump to form the sludge, predominantly consist of calcium sulfate.
An important use for this calcium sulfate is as gypsum, i.e. as a building material.
In the past, the desulfurization sludge drawn from the sump of such a scrubber and which can contain soluble impurities from the gas stream which are washed out by the scrubbing liquid was dewatered and the resulting solids washed in one or more washing stages with a washing liquid.
The washing liquid thus resulting or the liquid phase formed on dewatering contains these soluble impurities which are dissolved in the scrubbing liquid from the flue gas, the scrubbing liquid being generally water.
The liquid containing these impurities is subjected to a neutralization and as a result of this neutralization the impurities at least in part can be precipitated out and removed by filtering.
The dewatering and the subsequent washing process can be carried out in a centrifuge or in an assembly of centrifuges or on a vacuum belt filter. The impurities generally are silicates, carbonates, hydroxides, copper compounds, zinc compounds, and the like.
In German patent document-Open Application DE-OS No. 31 29 878, the washing liquid is neutralized after it has been recovered by dewatering or from the washing stage. The product is a neutralization sludge which has been discarded, e.g. by landfill deposition. This sometimes poses a disposal problem but the removal of the neutralization sludge from the liquid allows disposal of the latter without further specific treatment, e.g. by discharging into standard waste lines or as runoff.
In German patent document-Open Application DE-OS No. 32 05 636, the neutralization sludge is returned to the oxidation zone of the scrubbing column. While this has proved to be a partial answer to the disposal problem, it does not fully solve this problem because the return of the neutralization sludge to the scrubbing column and the recirculating of the sump liquid to the scrubbing zone results in an enrichment in the impurities of the sludge in the sump and which must be treated.
It should be apparent, therefore, that the very generation of a neutralization sludge which must be separately handled in various ways, as a product itself is disadvantageous.